Single axis force transducers are well known and are used in many situations in measuring or monitoring a load or force. When a structural member, such as that used for tractor lower draft links, is subjected to an axial force, the elastic deformation of the structural member can be measured by using bonded metallic resistance strain gauges to determine the magnitude of the force. For higher accuracy the strain gauge sensitivity can be increased by reducing the cross sectional area of the structural member in the strain gauge location.
When the structural member is subjected to a bending force, in additional to an axial force, such as in a tractor lower draft link, reducing the cross sectional area becomes more difficult and is generally a trade-off between transducer sensitivity and structural integrity. Unfortunately, the elastic deformation, due to bending, is several times greater than that due to tensile load, and therefore cross sensitivity of transverse forces, which are perpendicular to the axis of the structural member, introduces substantial error into the output signal.
In the past, the most common method of increasing the transducer sensitivity was by boring an aperture of substantial cross section through the central portion of the lower draft links as shwon in U.S. Pat. No. 3,246,701 issued April, 19, 1966. While this method does increase the sensitivity in the longitudinal axis of the lower draft links, it also increases the cross sensitivity of the transucer. For example, if the aperture in the lower draft link is between the tractor lift link and the implement secured to the lower draft link, and if an upwardly extending force is applied to the lift link in addition to the tensile force in the longitudinal direction of the lower draft link, material above the aperture will experience tensile stress, while the material below the aperture may experience compressive stress. An additional disadvantage of the aforementioned patent is that in order to achieve good sensitivity for tensile and compressive forces applied along the longitudinal axis of the lower draft link it is necessary to provide an aperture of relatively large diameter. This will materially reduce the strength of the lower draft links when bending loads are applied to the links.
The amount of resistance change to the strain gauge circuit, due to cross sensitivity, is dependent on the amount of force and the location of the strain gauge in relation to the neutral axis of the lower link in that plane. As a general rule, the further from the neutral axis that the strain gauge is located, the higher the stress that the strain gauge will experience.
In theory the strain gauge circuit can be designed so that the strain gauge resistance change, resulting from forces perpendicular to the axial or measuring direction, tend to offset each other and no net changes in bridge output is generated. However, this requires extreme accuracy in both machining and strain gauge placement. In some cases the only practical method of nullifying the extraneous signals is by using eight or more strain gauges instead of the usual four in the bridge circuit. Another element method used to isolate the force measuring element from side forces is to incorporate diaphragms or flexure webs connected to the force measuring element. Since such flexural supports not only take up side forces, but also part of the force to be measured, they have a negative influence on the measuring accuracy. These flexural supports also require precise machining.